More Inheritance and LSP CS340100, NTHU Yoshi

More about Inheritance Reuse? – Q1: 你有沒有程式 ” 砍掉重練 ” 的經驗 ? – Q2: 你有沒有 ” 再造輪子 ” 的經驗 ? class Rectangle – Firstly, we only have this design, and the system was developed class Square – Then, we have the requirement of Square, and we wrote a class Square – Square “IS A” kind of rectangle 1-2

Class Rectangle class Rectangle { double width; double height; public double getHeight() { return height; } public void setHeight(double height) { this.height = height; } public double getWidth() { return width; } public void setWidth(double width) { this.width = width; } } 1-3

Class Square class Square extends Rectangle { public void setHeight(double height) { super.setHeight(height); super.setWidth(height); } public void setWidth(double width) { super.setHeight(width); super.setWidth(width); } } 1-4

For Instance… void g(Rectangle r) { r.setWidth(5); r.setHeight(4); if (r.getWidth() * r.getHeight() != 20) { throw new RuntimeException(); } } //some other place Rectangle square = new Square(); g(square); 1-5

Liskov Substitution Principle (LSP) Functions that use pointers or references to base classes must be able to use objects of derived classes without knowing it Very difficult problem!! – Further study: OOAD – Software design principals Please google it if you are interested in it One of the principals – Prefer Composition over inheritance – Why? Can you give an example? 1-6

Delegation 1-7 Inheritance Aggregation